Hubble
works on the same principle as the first reflecting telescope
built in the 1600s by Isaac Newton. Light enters the telescope
and strikes a concave primary mirror, which acts like a lens
to focus the light. The bigger the mirror, the better the
image.

In Hubble, light from the primary mirror is reflected to a
smaller secondary mirror in front of the primary mirror, then
back through a hole in the primary to instruments clustered
behind the focal plane (where the image is in focus).

Angular
resolution
Hubble's angular resolution is 0.05 arcsecond. This is the
"sharpness" of Hubble's vision. If you could see as well as
Hubble, you could stand in New York City and distinguish two
fireflies, 1 m (3.3 feet) apart, in San Francisco.